1. Field of the Invention
The present invention relates to a photo-mask used in the exposure step of a process of making semiconductor elements and more particularly to such a photo-mask having a mark pattern for forming an alignment mark on a semiconductor wafer.
2. Description of the Prior Art
Semiconductor elements including LSI and the like are increasingly reduced in size and integrated with higher density. Exposure systems used in making semiconductor elements are thus required to provide more precise alignment. In such exposure systems, exposure is carried out when a mask or reticle is precisely placed over a semiconductor wafer on which a photosensitive agent or photoresist is applied. This requires detecting the position of an alignment mark on the wafer with high precision. Various proposals have been made with respect to the shape of the alignment mark and the detection of position of the same. One of various detection systems currently utilized can know the position of an alignment mark by detecting light information and particularly scattering light produced by the stepped edge of a linear mark. However, photoresist tends to be irregularly applied to the wafer at the stepped edges on the opposite sides of the linear mark. Such an irregurality of application causes light information from the stepped edges to be incorrect so that the precision in detection will substantially be decreased. In general, the application of photoresist is effected by the use of an apparatus known as a spinner that rotates a wafer at high speeds and drops liquid photoresist on the center of the wafer. The photoresist dropped on the wafer is spread toward the outer periphery of the wafer under the action of centrifugal force to form a film thereon. The thickness of the photoresist film is in the range of about 0.5 .mu.m to 2 .mu.m and depends on the rotational speed of the spinner, the viscosity of the photoresist used, etc. Such a method of applying the photoresist to the wafer can obtain a very uniform layer of photoresist through the entire surface of the wafer if there is no etched pattern on the wafer. If there is any uneven pattern on the surface of the wafer, however, the thickness of the photoresist layer becomes irregular near stepped edges of the pattern. When a raised or recessed mark line is desired, therefore, it may be better that the mark line be provided so as to extend radially from the center of the wafer. In such a case, the thickness of a photoresist layer formed becomes irregular at the stepped edges on the opposite sides of the mark line but substantially equally in irregularity. Thus, the central position of the mark line can accurately be detected when the positions of the opposite edges are detected. However, it is not really practicable to arrange all of mark lines to be formed on the wafer in accordance with such a rule. Particularly, where a plurality of alignment marks are formed on chips or dies on a wafer and each used to effect a two-dimensional alignment for each chip, the alignment marks may frequently extend perpendicular to the radial line from the center of the wafer or in a direction intersecting said radial line with a substantially large angle included therebetween, depending upon the positions of the respective chips on the wafer. Thus, the irregular thickness of the photoresist layer will become asymmetrical at the stepped edges on the opposite sides of each of the alignment marks. Consequently, the precision of detection will vary depending on the orientation of the respective alignment marks.